Hi Nhu
I sailed across the Atlantic from Grand Canaria to St Lucia via Cape Verde in the ARC+ 2018 and have experience in downwind ocean sailing up to F9 in a Seastream 43ft yacht displacing about 17 tons. The passage was 14 days from Cape Verde and we averaged about 140Nm a day (highest days run was 156Nm). We tried all the time to sail in F5 and above and largely achieved that. The vessel was cutter rigged, but the staysail was left unused on all down wind sailing – it is only good for upwind/reaching
Before leaving Gran Canaria I attended a whole series of seminars laid on by the ARC which included ocean sailing techniques and strategies.
We were told by the experts that the most stable means of sailing downwind is with twin poled out headsails. The poles needed to be secured with a topping lift and guys both fore and aft so that the poles are held absolutely rigid. This allows the genoa sheets to run freely to allow for furling. Twin poles means two topping lifts and four sets of guys. We lashed very large turning blocks to the aft cleats to allow a better run of the genoa sheets completely outboard of the vessel without guard wires getting in the way.
We did not have twin poles so we sailed under both main and genoa. Again here the experts were very explicit that the boom must have a fixed lines acting as boom preventers (NOT a gybe preventer you can buy – they break) attached to the aft end of the boom at all times. We permanently rigged two boom preventers from the clew leading down each side of the boat to the bow and then led aft to the cockpit. It was then a matter of cleating the appropriate preventer depending on which tack we were on. The experts said that if you rig the preventer part way along the boom there is a danger of folding the boom in half. One vessel (60ft) ignored this advice and broke their boom. Breakage occurs during an accidental gybe, which is almost inevitable sailing downwind in a big sea.
Sailing downwind under main and genoa causes issues with reefing. To reef or unreef the main you need head up to wind sufficiently to allow the sail go up and down the mast. We found this meant furling the genoa completely, and then heading slightly upwind, reefing, or unreefing, steering back on course and unfurling the genoa again. It took about half an hour.
Similarly gybing meant furling the genoa, moving the pole across to the other side, rigging it in position before gybing, hauling in the main sheet more or less completely, gybing, letting the main sheet out, re-rigging the preventer and then unfurling the genoa. Took about 3/4 hour.
The only other point to remember is that the loads on all the standing and running rigging are absoletely enormous. It is worth checking all standing rigging, especially the fittings and swages for cracks, even if the rigging has been recently replaced. This will mean an extensive and close look up the mast in the bosuns chair. One vessel on the ARC+2018 suffered a broken forestay masthead fitting, it had been replaced only 6 months earlier.
As mentioned already the loads are enormous, We lashed the wheel with rope to eyebolts in the cockpit to keep the rudder fixed and did not trust the helm lock. We steered almost exclusively using the hydrovane. We did have an autopilot but decided this could not withstand the massive loads.
I hope this helps.